The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
The power train in a conventional rear wheel drive vehicle includes a rear axle or differential having a gear set which changes the power flow rotational axis from longitudinal to transverse and four caged bevel gears which allow the wheels and axles to rotate at different speeds when, for example, the vehicle is turning. Many four wheel drive vehicles also utilize a differential in the front axle for the same purpose.
Maintaining proper lubrication in the rear axle or differential, particularly the hypoid gear set, is both critical and challenging. Lubrication of the hypoid gear set is essentially achieved by rotation of the ring gear through the lubricating fluid which fills the lower portion of the differential housing. Lubricating fluid is carried by and between the gear teeth and into the region of mesh with the pinion or worm gear. The speed of the ring gear and the viscosity of the lubricating fluid determine how much lubricating fluid will be carried by the ring gear and thus available to lubricate the mesh as well as how vigorously the fluid will circulate within the axle housing.
Since the speed of rotation is directly related to vehicle speed, it is essentially an uncontrolled variable. The viscosity of the lubricating fluid is related to its temperature and this may vary significantly in the course of vehicle operation. The viscosity of the lubricating fluid should be high enough to form a film at the mesh thick enough to separate mating surfaces to avoid scoring or abrasive wear. Higher fluid viscosities are thus preferable as they ensure that more fluid will be carried by the ring gear teeth to the mesh. Viscosities that are too high, however, contribute to frictional and churning losses which can account for a significant portion of the energy loss in a differential. The problem of energy loss due to high viscosity is particularly acute during start ups in cold environments.
From the foregoing, it is apparent that improvements in rear axles and differentials to provide improved gear lubrication and reduced frictional losses through improved viscosity control would be desirable.